<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Test01</title>
    <script src="https://unpkg.com/vue/dist/vue.js" type="text/javascript"></script>
</head>
<body>

<div id="app">
    <p>{{ message }}</p>
</div>

<script>
    let vm = new Vue({
        el: '#app',
        data: {
            message: 'Hello Vue!'
        }
    });

    vm.message = "Hello Vue!!"

</script>

</body>
</html>